Regulation E doesn't provide for that type of dispute. If the sales slip said $800 and the debit came through for $900, there'd be a claim for wrong amount. And clearly she can't claim that the debit was unauthorized. So Regulation E is not involved.
Visa, however, might have a provision for a claim concerning the quality of the service provided. You'll need to review the Visa claims provisions in your Visa Operating Rules to determine whether their is such a provision, and how to proceed. Otherwise, the matter may be something the customer has to work out with the service provider.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8